Method of forming quantum dots for extended wavelength operation

ABSTRACT

A method of forming the active region of an optoelectronic device incorporating semiconductor quantum dots whose ground state emission occurs at wavelengths beyond 1350 nm at a temperature of substantially 293 K is provided by forming a first layer of quantum dots covered by a spacer layer with strained areas extending there through. The spacer layer then forms a template upon which quantum dots of an active layer may be formed with a surface with a surface density and formation that is influenced by the underlying first layer of quantum dots. This allows a choice of growth parameters more favourable to the formation of quantum dots in the active layer emitting at long wavelengths with a narrow inhomogeneous broadening. As an example, the active layer of quantum dots may be formed at a lower temperature than the first layer of quantum dots. The quantum dots of the active layer are then subject to less intermixing with the surrounding spacer and capping layers, and can also preserve a more strain-relaxed state, which results in a longer wavelength emission with a narrower inhomogeneous broadening. This method is particularly well suited to the growth of the active region of an optoelectronic device on a GaAs substrate.

This invention relates to the field of optoelectronic devices. Moreparticularly, this invention relates to the field of optoelectronicdevices incorporating semiconductor quantum dots whose ground stateemission occurs at wavelengths greater than 1350 nm at a temperature ofsubstantially 293 K.

Semiconductor materials are used in many optoelectronic devices. Thesemiconductor structure is normally arranged in order that the device isoptically active at a wavelength desired for that particular device. Inmany applications, particularly in telecommunications, there is arequirement to use wavelengths between 1250 and 1650 nm. Thesewavelengths are well suited to fibre optic transmission and to otherfibre optic devices.

It is known to manufacture optoelectronic devices for use at thesewavelengths based upon Indium Phosphide (InP) substrates. It would bedesirable to be able to fabricate devices operable at these wavelengthsusing Gallium Arsenide (GaAs) substrates rather than InP substrates.

The main advantages of GaAs substrates are that they are widelyavailable and less expensive than InP substrates. GaAs substrates arealready widely used for devices operating at shorter wavelengths (below1200 nm) and the methods of fabricating, processing and packaging suchdevices are well developed and could be adapted to devices operating ata longer wavelength.

The performances of GaAs-based devices are in some respects superior tothose based upon InP, especially in terms of temperature sensitivity.Complex structures, such as Vertical Cavity Surface Emitting Lasers, canbe often readily fabricated on GaAs systems in a single growth step,compared with the complicated processes, such as wafer bonding, thatwould be required for InP systems to achieve the same structures.Furthermore, GaAs electronics is well developed and GaAs-basedoptoelectronic devices integrating monolithically on the same chip boththe optical functions and the electronic circuitry required to controlthem may be relatively readily provided.

It will be appreciated from the above that if GaAs systems could be madeto operate at longer wavelengths, then this would be highly desirable.

There are three known technologies which have been shown to achievelonger wavelength operation on GaAs substrate: (see for example V. M.Ustinov and A. E. Zhukov, “GaAs-based long-wavelength lasers”, Semicond.Sci. Technol. 2000, 15, R41). They are InAs or InGaAs quantum dots,GaInNAs quantum wells or dots, and GaAsSb quantum wells.

In the case of GaInNAs and GaAsSb quantum wells, the addition ofNitrogen or Antimony in the structure reduces the band gap and leads tolonger wavelength emission. However, with the current growth techniques,the quality of the material is degraded when incorporating Nitrogen orAntimony.

Moreover, quantum dots present several advantages over the two competingtechniques (based on quantum wells). These advantages are due to thethree dimensional confinement of carriers (as opposed to one-dimensionalfor quantum wells) and to the inhomogeneously broadened emission (asopposed to homogeneously broadened emission for quantum wells). Suchadvantages are lower threshold current for lasers, lower temperaturesensitivity, or the possibility of operating on a broader band ofwavelengths.

Another important feature is that quantum dots can usually operate notonly in their fundamental transition, but also in a wide band ofwavelengths (corresponding to the excited states) shorter than theirfundamental transition. For example, producing quantum dots emitting at1480 nm from their ground state is not only important for applicationsat this wavelength, but can also be used at shorter wavelengths such as1300 nm (from their second excited state, for example). Some benefitsare then gained compared to quantum dots emitting directly light at 1300nm from their ground state, due to an increased degeneracy of theexcited states.

InAs or InGaAs quantum dots are usually fabricated according to theStranski-Krastanov growth mode, whereby the strain resulting from alattice mismatch between the substrate (GaAs) and the dot material (InAsor InGaAs) leads to the self-formation of three-dimensional islands. Thegrowth is usually achieved using two main techniques: Molecular BeamEpitaxy (MBE) or Metal Organic Chemical Vapour Deposition (MOCVD). Underconventional growth conditions (i.e. similar to what is used for InGaAsquantum wells), the lateral dot dimensions are typically between 14 and30 nm (see for example U.S. Pat. No. 5,614,435) and they typically emitat wavelengths shorter than 1200 nm at 300K. In the cases presented herefor longer wavelength emission, the lateral dot dimensions are typicallylarger.

To extend the wavelength of InAs/GaAs quantum dots further, differenttechniques have been developed: R. P. Mirin et al., “1.3 μmphotoluminescence from InGaAs quantum dots on GaAs”, Appl. Phys. Lett.1995, 67, 3795 proposed using Alternate Layer Epitaxy. R. Murray et al.,“1.3 μm room temperature emission from InAs/GaAs self-assembled quantumdots”, Jpn. J. Appl. Phys. 1999, Part 1 38, 528 proposed using low InAsgrowth rates. However, the longest wavelengths achieved with thesetechniques are close to 1300 nm and at most 1340 nm.

An alternative method involving either capping the dots with InGaAs, orgrowing the dots on InGaAs, or both was developed. K. Nishi et al., “Anarrow photoluminescence linewidth of 21 meV at 1.35 μm fromstrain-reduced InAs quantum dots covered by In _(0.2)Ga_(0.8) As grownon GaAs substrates”, Appl. Phys. Lett. 1999, 74, 1111 proposed cappingthe dots with InGaAs and achieved emission up to 1350 nm with MBE. A.Passaseo et al., “Wavelength control from 1.25 to 1.4 μm in In _(x) Ga_(1-x) As quantum dot structures grown by metal organic chemical vapordeposition”, Appl. Phys. Lett. 2001, 78, 1382 used a similar techniqueto achieve emission up to 1390 nm with MOCVD. Finally, J. Tatebayashi etal., “Over 1.5 μm light emission from InAs quantum dots embedded inInGaAs strain-reducing layer grown by metalorganic chemical vapordeposition”, Appl. Phys. Lett., 2001, 78, 3469 observed emission up to1520 nm, but with a strongly reduced luminescence efficiency, which istherefore not suitable for use in an optoelectronic device.

Another important property of a quantum dot sample is the inhomogeneousbroadening of the emission, measured by the Full Width at Half Maximum(FWHM) of the ground state photoluminescence peak at low excitation andlow temperature (typically 10 K). For many applications such as lasers,the FWHM needs to be as small as possible for the best performances. Thenarrowest FWHM achieved for emission wavelengths above 1300 nm is 18meV, in R. P. Mirin et al., “Narrow photoluminescence linewidths fromensembles of self-assembled InGaAs quantum dots”, J. Vac. Sci. Technol.B 2000, 18, 1510.

One feature of the growth of quantum dots relevant to the presentinvention is the possibility to grow vertically aligned quantum dotstructures simply by growing successive closely spaced quantum dotlayers. This feature was recognized early (see for example Q. Xie etal., “Vertically self-organized InAs quantum box islands on GaAs (100)”,Phys. Rev. Lett. 1995, 75, 2542) and has been much studied since.Mukhametzhanov et al., <<Independent manipulation of density and size ofstress-driven self-assembled quantum dots>>, Appl. Phys. Lett. 1998, 73,1841 used this feature to grow a second layer of larger dots with alower density than would otherwise be possible with the growthconditions used. A first layer with a low density of small quantum dotswas grown which determined the density of the second layer, due to thevertical stacking. The resulting quantum dots in the second layer,although grown at a conventional growth rate of 0.22 ML/s were thensimilar (in terms of density, dimensions and emission properties) toquantum dots directly grown at a low growth rate.

Viewed from one aspect the present invention provides an optoelectronicdevice including semiconductor quantum dots capable of at least one ofemitting, absorbing or amplifying radiation in their ground state atwavelengths greater than 1350 nm at a temperature of substantially 293K,or in their excited states.

Viewed from another aspect the present invention provides a method offorming the active region of an optoelectronic device incorporatingsemiconductor quantum dots whose ground state emission occurs atwavelengths greater than 1350 nm at a temperature of substantially 293K, said method comprising the steps of:

-   -   growing a first layer of quantum dots formed on one of a        substrate layer or a buffer layer, said quantum dots of said        first layer being subject to a strain due to a lattice mismatch        between said substrate layer and said quantum dots of said first        layer;    -   growing a spacer layer over said first layer and said spacer        layer being subject to a strain in strained areas overlying        quantum dots of said first layer due to a lattice mismatch        between said quantum dots of said first layer and said spacer        layer;    -   growing an active layer of quantum dots on said spacer layer,        quantum dots of said active layer predominately forming upon        strained areas of said spacer layer such that the surface        density of quantum dots of said active layer is substantially        determined by the surface density of quantum dots of said first        layer, quantum dots of said active layer being in a        strain-relaxed state in which said quantum dots of said active        layer are subject to less strain than quantum dots grown on an        unstrained surface, growth conditions for said active layer        being different to those of the first layer and chosen        appropriately, in particular substrate temperature being low        enough, such as to substantially preserve said strain-relaxed        state and to limit intermixing of said quantum dots of said        active layer with said spacer layer; and    -   growing a capping layer on said active layer, growth conditions        for said capping layer chosen appropriately, in particular        substrate temperature being low enough, such as to substantially        preserve said strain-relaxed state and to limit intermixing of        said quantum dots of said active layer with said spacer layer        and with said capping layer.

The present invention relies in part on the possibility of growing afirst layer to set the density of a second layer. Although thistechnique has been known for many years, the possibility to use thefirst layer to engineer the strain state of the second layer, and theimportance of the intermixing effects during growth and capping of thissecond layer were not recognized. This is why this technique has notbeen used to extend the emission of the quantum dots to desirablewavelengths beyond 1350 nm at room temperature.

It will be understood that this technique is particularly well suited toembodiments where the substrate is a GaAs substrate.

Under suitable growth and capping conditions, the strain relaxation inthe quantum dots of the active layer leads to a lower band gap andconsequently a ground state emission at a longer wavelength. The cappingconditions may be chosen such that the benefits obtained from thestrain-relaxation (long wavelength emission) are not lost due to anothercompeting mechanism. For example, strain-relaxed InGaAs quantum dotshave a tendency to experience more gallium/indium intermixing duringcapping, which would result in a shorter wavelength emission. Thesubstrate temperature therefore can be made low enough to avoid theseintermixing effects. This is facilitated by the fact that the surfacedensity of the quantum dots of the active layer is determined by that ofthe first layer, due to the strain interaction. The growth parameters ofthe active layer can therefore be adjusted without affecting its quantumdot density. This is different from a conventional quantum dot layer,where changing the growth parameters strongly affects the density, andwhere a reduced substrate temperature, for example, leads to a highdensity of small quantum dots emitting at a short wavelength.

Existing techniques which have been used to extend the emissionwavelength of InAs quantum dots, such as the use of InGaAs barriers orthe incorporation of Nitrogen, are usually associated with a degradationof material quality and consequently strong reductions in the intensityof emission at room temperature. These techniques may be advantageouslyavoided in some embodiments whilst maintaining a long wavelength andstrong room temperature emission.

The strain interaction between layers and the reduced intermixing alsoproduces quantum dots with a better uniformity. Much narrower FWHM inthe photoluminescence emission of the active layer can therefore beachieved.

It will be appreciated that the thickness of the spacer layer could varydepending upon the particular circumstances and substances being used,but this spacer layer has been found to advantageously have a thicknessof 3×10⁻⁹ m to 3×10⁻⁸ m.

In the preferred embodiments, it is advantageous that the quantum dotsof the first layer are grown such as their strain field is strong enoughto extend substantially through the spacer layer. This is facilitated bygrowing the first quantum dot layer at a low growth rate of less than0.06 monolayer per second. For convenience, this growth rate may also bekept unchanged for the second quantum dot layer.

It will be appreciated that, depending on the spacer layer thickness,the quantum dots of the active layer may be electronically coupled tothose of the first layer, which can represent an advantage for someapplications.

Whilst it will be appreciated that depending upon the particular deviceand application, the surface density of the quantum dots could varyconsiderably, the invention is particularly well suited to embodimentsin which the surface density of the quantum dots is between 10¹³ and10¹⁵ per square meter.

It will be understood that for some devices, and in particularcircumstances, it may be desirable to have more than one active layer ofquantum dots and such can be achieved by forming further spacer layers(possibly provided by the capping layer) and active layers to produce astack of active layers of quantum dots having the desirable properties.

Whilst the present techniques may be applied using a range of differentmaterial possibilities, in preferred embodiments the quantum dots areone of InAs quantum dots, InGaAs quantum dots or GaInNAs quantum dots.In a similar way at least part of the substrate layer could be formed ofa variety of materials, but is preferably one of GaAs or AlGaAs. Thespacer layer and the capping layer may also be at least partially formedof a variety of different complementary materials, such as GaAs, AlGaAs,in GaAs, InAlGaAs or GaInNAs. In particular, it can be advantageous toform the last part of the spacer layer or the first part of the cappinglayer or both with InGaAs instead of GaAs.

The optoelectronic devices formed using the above described techniquescould have a wide variety of different functions and forms dependingupon the particular application, but the present invention isparticularly useful when the active layer is operable to perform atleast one of radiation emitting, radiation amplifying, radiationdetecting and radiation absorbing.

A first feature of the active region according to the current techniqueand associated with its improved performances is that the density ofdots in the active layer is determined by the density of dots of thefirst layer. This enables choice of the density of the dots in theactive layer independently of the growth parameters used to grow and capit.

A second feature of this technique is that the first feature is used tochoose the growth parameters of the active layer such that thestrain-relaxed state of the dots of the active layer is preserved andthat the intermixing effects are reduced as much as possible. These aretwo significant factors in achieving a longer wavelength emission with anarrow broadening.

A particularly preferred advantageous feature of this method of formingan optoelectronic device is that the active layer is grown at a lowertemperature than the first layer. This is facilitated by thetemplate/keying action of the strained areas of the spacer layer inproviding sites for the quantum dots of the active layer to form attemperatures lower than would otherwise be required to form such quantumdots in the absence of the spacer layer. The formation of the quantumdots of the active layer at a lower than otherwise temperature tends toimprove their uniformity and performance characteristics, e.g. emissionat a longer wavelength with a narrower FWHM.

The action of the spacer layer in providing a template/key for formationof quantum dots in the active layer is improved when the spacer layer issubject to annealing prior to the formation of the quantum dots of theactive layer.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates the formation of the active region ofan optoelectronic device;

FIG. 2 schematically illustrates the layer structure of the activeregion of an optoelectronic device having five active layers of quantumdots;

FIG. 3 schematically illustrates the layer structure of the activeregion of an optoelectronic device having three active layers of quantumdots;

FIG. 4 illustrates the low temperature, low excitation photoluminescencespectrum of the active region of an optoelectronic device where thespacer and capping layers are composed of GaAs only;

FIG. 5 illustrates the room temperature, high excitationphotoluminescence spectrum of the same active region of anoptoelectronic device as that shown in FIG. 4;

FIG. 6 illustrates the low temperature, low excitation photoluminescencespectrum of another active region of an optoelectronic device where thelast part of the spacer layer and the first part of the capping layerare composed of InGaAs; and

FIG. 7 illustrates the room temperature, high excitationphotoluminescence spectrum of the same active region of anoptoelectronic device as that shown in FIG. 6.

There is described a method of producing high quality quantum dotlayers, emitting at longer wavelengths (above 1350 nm) from their groundstate, with a good luminescence efficiency and a narrow FWHM. Such astructure can be used as the active region of many optoelectronicdevices grown on GaAs substrates and operating at wavelengths above 1350nm. Such an active region could also provide significant improvements tooptoelectronic devices operating at shorter wavelengths, by usingemission and absorption in the excited states of the dots.

FIG. 1 schematically illustrates the formation of the active region ofan optoelectronic device. At step (a) a substrate, such as a GaAssubstrate, is provided, typically in the form of a wafer. A bufferlayer, such as GaAs, can be grown on this substrate.

At step (b), a first layer of quantum dots, such as InAs quantum dots,is formed according to the Stranski-Krastanov growth mode whereby thestrain resulting from the lattice mismatch between the substrate and thequantum dots results in the self-formation of three-dimensional islandswhich constitute the quantum dots.

At step (c), a spacer layer, such as a GaAs spacer layer, is depositedon top of the first layer. The lattice constant mismatch between thequantum dots of the first layer and the spacer layer results in a strainfield between the spacer layer and the quantum dots which extends upthrough to the top surface of the spacer layer to produce strained areason the top surface of the spacer layer corresponding to the underlyingquantum dots of the first layer.

At step (d), an active layer of quantum dots, such as InAs quantum dots,is deposited on top of the spacer layer. The strained areas on the topsurface of the spacer layer form a template for the formation of thequantum dots of the active layer which are aligned to overlie thequantum dots of the first layer. The presence of this strain fieldusually manifests itself by a reduction in the InAs coverage requiredfor the growth to change from 2D to 3D, as can be measured usingReflection High Energy Electron Diffraction (RHEED). The surface densityof the quantum dots of the active layer is effectively controlled by thetemplate provided by the strained areas on the top surface of the spacerlayer rather than by the particular growth parameters used for theformation of the active layer. Accordingly, the active layer of quantumdots can be formed with different growth parameters than would otherwisebe required to form that active layer of quantum dots in the absence ofthe underlying first layer and spacer layer. This at least partialdecoupling of the growth parameters for the quantum dots of the activelayer from the quantum dot surface density of the active layer allowsgrowth parameters to be used which are more favourable to achieving longwavelength emission characteristics of the active layer of quantum dotsthan would otherwise be possible. As an example, the active layer ofquantum dots may be grown at a lower temperature resulting in lessintermixing of the material of the quantum dots with the underlyingspacer layer. The higher uniformity of the active layer of quantum dotsthis formed results in a narrower inhomogeneous broadening. Furthermore,since the spacer layer is strained by the underlying first layer, it hasa lattice constant in the strained areas that more closely matches thequantum dots of the active layer and accordingly the quantum dots of theactive layer are more strain-relaxed than quantum dots forming on anunstrained surface. This, together with the reduced intermixing,contributes to the desirable long wavelength emission.

At step (e), a capping layer, such as GaAs, is formed over the activelayer of quantum dots. The capping layer, and especially its first fewnanometers, is usually, but not necessarily, grown under the same growthconditions as the active layer to avoid a growth interruption.

It will be appreciated that the simple example shown in FIG. 1 uses onematerial system, namely GaAs and InAs, but that other possibilities areavailable, such as forming the capping layers and/or spacer layers, orpart of them, of InGaAs instead of GaAs. As further alternatives,material systems such AlGaAs with AlGaAs spacers may be used in place ofthe GaAs and InGaAs layers discussed above. GaInNAs may also be used asthe material of the quantum dots or spacer/capping layers.

The FIG. 1 embodiment shows the formation of a single active layer. FIG.2 illustrates the active region of an optoelectronic device in whichfurther active layers are formed by repeatedly depositing a spacer layerand an active layer. In the FIG. 2 example, once the initial first layerof quantum dots has been formed, five groups of associatedcapping/spacer layers and active layers are deposited thereupon followedby a final capping layer.

FIG. 3 illustrates and alternative embodiment of the active region of anoptoelectronic device. In this embodiment three active layers areformed. The device is formed of three groups of layers, each group oflayers comprising a first layer, a spacer layer, an active layer and acapping layer. After the first capping layer has been deposited, anotherfirst layer is deposited thereupon and the sequence repeated.

A distinction between a capping layer and a spacer layer in the FIG. 3embodiment relates to the thickness of the capping layer relative to thethickness of a spacer layer. A capping layer will generally be muchthicker, so that the strained areas do not extend up through it.

FIG. 4 illustrates the low temperature (10K), low excitationphotoluminescence spectrum of the active region of an optoelectronicdevice formed in accordance with the above techniques. In this firstexample, the spacer layer and the capping layer are formed of GaAs only.As will be seen, the emission peaks at a relatively high 1280 nm with anarrow full width and a half maximum of 14 meV indicating the highuniformity of the quantum dots that have been formed.

FIG. 5 illustrates the room temperature, high excitation spectrum forthe same device. In this case, it will be seen that the emission peakshave moved up in wavelength for this room temperature operational regimeof the device to approximately 1390 nm for the ground state emission,which even extends up to 1430 nm. This makes possible the fabrication ofGaAs based devices operating at wavelengths beyond 1350 nm. Moreover,the first excited state emission is observed around 1300 nm. Such anactive region could therefore improve the performance of GaAs basedquantum dot devices operating near 1300 nm, since the maximum opticalgain obtainable from the first excited state is twice as large as thatobtainable from the ground state.

Another example of a quantum dot sample illustrating the benefits ofthis technique is now given. The difference with the previous example isthat InGaAs was used to cap the dots instead of GaAs. The resultingemission wavelength is even longer and observed at 1480 nm at roomtemperature.

FIG. 6 illustrates the low temperature (10K), low excitationphotoluminescence spectrum of this other active region formed inaccordance with the above techniques. The difference with the previousexample is that InGaAs was used in the last part of the spacer layer andin the first part of the capping layer, instead of GaAs. As will beseen, the emission peaks at an even higher wavelength of 1350 nm (at10K) with a narrow FWHM of 14.5 meV.

FIG. 7 illustrates the room temperature, high excitation spectrum forthe same active region. In this case, it will be seen that roomtemperature emission from the ground state peaks at 1480 nm, and extendsbeyond 1500 nm. This again makes possible the fabrication of GaAs-baseddevices operating at these long wavelengths. Also, first excited stateemission is obtained at 1390 nm and second excited state emission around1300 nm. Such an active region could therefore improve the performanceof GaAs based quantum dot devices operating at these wavelengths, sincethe maximum optical gain obtainable from the second excited state isfour times that of the ground state, and that of the first excited stateis twice as large as that from the ground state.

A more specific example of the parameters used to form the exampleoptoelectronic device for which the low temperature and room temperaturespectra are illustrated in FIGS. 4 and 5 is given below.

A 2 inch n+ doped GaAs wafer is loaded in a molecular beam epitaxy (MBE)system and pumped down to an ultra high vacuum. The substratetemperature is taken to 620° C. for oxide removal. After growth of a 100nm GaAs buffer layer at 580° C., the temperature is dropped to 500° C.for growth of the first quantum dot layer. The absolute values of thesubstrate temperatures are difficult to evaluate in an MBE system. Inour case, the reference temperature is the temperature for which theGaAs surface reconstruction changes from a 2×4 pattern to a c4×4pattern, this pattern being monitored with Reflection High EnergyElectron Diffraction (RHEED). For the Arsenic background pressure usedhere (2.6×10⁻⁶ mbar), this reference temperature was measured to be 500°C. with a pyrometer. The other temperatures are measured relative tothis reference using a thermocouple. The first quantum dot layerconsists of deposition of 2.2 ML of InAs at a growth rate of 0.015monolayer per second, and immediately capped with 12 nm of GaAs at 500°C. The temperature is then raised to 580° C., the sample annealed for 10minutes, and the temperature dropped back to 470° C. for growth of theactive quantum dot layer. This layer consists of deposition of 2.7 ML ofInAs at 470° C., immediately capped with 10 nm of GaAs at 470° C. Thetemperature is then raised to 580° C. for the growth of the remainder ofthe GaAs cap (100 nm). See FIGS. 4 and 5 for low temperature and roomtemperature photoluminescence from such a sample.

For the second example (FIGS. 6 and 7), the only differences are thatthe spacer layer consists of 10 nm of GaAs followed by 2 nm of InGaAswith an indium composition of 15% and that the first 5 nm of the cappinglayer are replaced by 5 nm of InGaAs with an indium composition of 26%.

Key to Figures FIG. 4

Low temperature, low excitation photoluminescence spectrum. The emissionpeaks at 0.968 eV (1280 nm) with a narrow full width at half maximum of14 meV indicating the high uniformity of the quantum dots.

FIG. 5

Room temperature, high excitation spectrum. The ground state emissionpeaks at 1390 nm and extends up to around 1430 nm. First excitedemission occurs around 1300 nm

FIG. 6

Low temperature, low excitation photoluminescence spectrum of the secondexample (containing InGaAs in the spacer and capping layers). Theemission peaks at 0.918 eV (1350 nm) with a narrow full width at halfmaximum of 14.5 meV indicating the high uniformity of the quantum dots.

FIG. 7

Room temperature, high excitation photoluminescence spectrum of thesecond example (containing InGaAs in the spacer and capping layers). Theground state emission peaks at 1480 nm and extends beyond 1500 nm. Firstexcited state emission occurs at 1390 nm and second excited stateemission around 1300 nm.

1. A method of forming the active region of an optoelectronic deviceincorporating semiconductor quantum dots whose ground state emissionoccurs at wavelengths greater than 1350 nm at substantially 293 K, saidmethod comprising the steps of: growing a first layer of quantum dotsformed on one of a substrate layer or a buffer layer, said quantum dotsof said first layer being subject to a strain due to a lattice mismatchbetween said substrate layer and said quantum dots of said first layer;growing a spacer layer over said first layer and said spacer layer beingsubject to a strain in strained areas overlying quantum dots of saidfirst layer due to a lattice mismatch between said quantum dots of saidfirst layer and said spacer layer; growing an active layer of quantumdots on said spacer layer, quantum dots of said active layerpredominately forming upon strained areas of said spacer layer such thatthe surface density of quantum dots of said active layer issubstantially determined by the surface density of quantum dots of saidfirst layer, quantum dots of said active layer being in a strain-relaxedstate in which said quantum dots of said active layer are subject toless strain than quantum dots grown on an unstrained surface, growthconditions for said active layer being different to those of the firstlayer and chosen appropriately, in particular substrate temperaturebeing low enough, such as to substantially preserve said strain-relaxedstate and to limit intermixing of said quantum dots of said active layerwith said spacer layer; and growing a capping layer on said activelayer, growth conditions for said capping layer chosen appropriately, inparticular substrate temperature being low enough, such as tosubstantially preserve said strain-relaxed state and to limitintermixing of said quantum dots of said active layer with said spacerlayer and with said capping layer.
 2. A method as claimed in claim 1,wherein said spacer layer is grown to a thickness of 3×10⁻⁹ m to 3×10⁻⁸m.
 3. A method as claimed in claim 1, wherein said first layer ofquantum dots is grown at a growth rate of less than 0.06 monolayer persecond.
 4. A method as claimed in claim 1, wherein said quantum dots insaid first layer are grown to have a surface density of between 10¹³ and10¹⁵ per square meter.
 5. A method as claimed in claims 1, wherein saidcapping layer acts as a spacer layer for growth of one or more furtheractive layers and capping layers.
 6. A method as claimed in claim 1,comprising growing one or more further first layer, spacer layer, activelayer and capping layer groups on said capping layer.
 7. A method asclaimed in claim 1, wherein said quantum dots are one of: (i) InAsquantum dots; (ii) InGaAs quantum dots; and (iii) GaInNAs quantum dots.8. A method as claimed in claim 1, wherein at least part of saidsubstrate layer or said buffer layer is one of: (i) GaAs; (ii) AlGaAs.9. A method as claimed in claim 1, wherein at least part of said spacerlayer is one of: (i) GaAs; (ii) AlGaAs; (iii) InGaAs; (iv) InAlGaAs; and(v) GaInNAs.
 10. A method as claimed in claim 1, wherein at least partof said capping layer is one of: (i) GaAs; (ii) AlGaAs; (iii) InGaAs;(iv) InAlGaAs; and (v) GaInNAs.
 11. A method as claimed in claim 1,wherein said active layer is operable to perform at least one of: (i)radiation emitting; (ii) radiation amplifying; (iii) radiationdetecting; and (iv) radiation absorbing.
 12. A method as claimed inclaim 1, wherein the mean size of quantum dots in said active layer isdifferent to the mean size of quantum dots in said first layer.
 13. Amethod as claimed in claim 1, wherein said active layer is grown at alower temperature than said first layer.
 14. A method as claimed inclaim 1, wherein said capping layer is grown at a lower temperature thansaid first layer.
 15. A method as claimed in claim 1, wherein saidspacer layer is annealed prior to growing said active layer on saidspacer layer.
 16. A method as claimed in any one of the preceding claims1, wherein growth is interrupted between said spacer layer and saidactive layer in order to change the growth parameters.
 17. A method asclaimed in claim 1, wherein growth is interrupted between said activelayer and said capping layer in order to change the growth parameters.18. A method as claimed in claim 1, wherein the quantum dots of saidfirst layer are electronically coupled to the quantum dots of saidactive layer.
 19. A method as claimed in claim 1, wherein said quantumdots of said active layer are operable to at least one of emit, absorbor amplify light in their ground states.
 20. A method as claimed in anyclaim 1, wherein said quantum dots of said active layer are operable toat least one of emit, absorb or amplify light in their excited states.21. An optoelectronic device containing an active region grown accordingto the method described in claim 1.